There is a silicon single crystal, for example, as a single crystal used as a substrate of a semiconductor device, and the silicon single crystal is produced mainly by Czochraiski method (hereinafter abbreviated as CZ method).
When a single crystal is produced by CZ method, single crystal production equipment 30 shown in FIG. 8, for example, is used. This single crystal production equipment has a main chamber 31 in which a crucible is placed at the center thereof. The crucible has a double structure and is formed of a quartz crucible 36 and a graphite crucible 37, which holds the outside of the quartz crucible.
These crucibles are secured to the upper end of a shaft 38 in such a way that the crucibles can rotate and move up and down, and a graphite heater 39 is placed outside the crucible. Furthermore, insulating material 40 is placed concentrically around the outside of the graphite heater. In addition, silicon melt 35, which is silicon raw material melted by the graphite heater, is contained the quartz crucible.
Moreover, a wire 34 which rotates at a predetermined rate on the same axis as the shaft in a direction opposite to the shaft or in the same direction as the shaft is placed on the central axis of the quartz crucible filled with the silicon melt, and a seed crystal 45 is held at the lower end of the wire. In addition, a single crystal 33 is formed at a lower end face of the seed crystal.
Furthermore, a cooling cylinder 41 to cool the single crystal which has been pulled upwardly and, in the lower part thereof, a graphite cylinder 42 are provided, whereby the single crystal which has been pulled upwardly can be cooled by passing coolant gas through the cylinders downward. In addition, a thermal shield 43 is provided outside the lower end of the graphite cylinder to intercept radiation from the surface of the melt and keep the heat on the surface of the silicon melt.
Moreover, equipment in which, in addition to a graphite heater, a heater for melting silicon raw material is placed in order to shorten the single crystal production time has been disclosed (see, for example, Japanese Patent Laid-Open (kokai) No. H6-183876). However, since this heater is placed immediately above the silicon melt in a position above the graphite heater, dislocation is undesirably generated in the single crystal as a result of dust falling on the single crystal during the growth of the single crystal. Furthermore, when the SiO gas evaporated from the silicon melt attaches to this upper heater, the graphite gradually degrades due to silicification. This undesirably increases the carbon concentration in the single crystal.
In addition, a method for controlling a crystal defect by controlling the ratio of power between the upper and lower heaters by providing a graphite heater with a two-stage structure as shown in FIG. 9 and placing, above a conventional graphite heater 39, a cylindrical heater 44 having the same inside and outside diameters as those of the graphite heater 39 has also been disclosed (see, for example, Japanese Patent Laid-Open (kokai) No. 2001-261482).
In this case, the specific resistance of graphite is 1000 to 1500 μΩcm, which is extremely low, at room temperature and it is hard to produce heat in this state, it is necessary to form a zigzag current path by forming slits in several points in the upper and lower edges in a circumferential direction. However, when a static magnetic field is applied by a superconducting coil or the like, the heater is deformed by the Lorentz force. Therefore, it is necessary to prevent deformation by maintaining strength and thereby prevent discharge between the heater and the other graphite parts. As a result, there are limitations to shorten drastically the length of the upper and lower ends in which the slits do not cross each other.
Thus, when the heat generation center of the upper heater is placed near a growth interface of the single crystal, the lower edge of the upper heater is lower than the melt surface, and the position of the lower heater is lower than the silicon melt as compared to the single-stage heater shown in FIG. 8. This makes it difficult to perform control as a result of an increase in oxygen concentration in the crystal. Moreover, the former half of a straight body has large G, and it is difficult to control the value of V/G so as to be a value at which target crystal quality can be obtained.
Here, V/G is a ratio between a pulling rate V and a crystal solid-liquid interface temperature gradient G and is a parameter which can control two types of point defects, vacancies and interstitial silicon, and has received attention as a control factor of Grown-in defects and oxygen precipitation characteristics. By adjusting the pulling rate V and the crystal solid-liquid interface temperature gradient G so as to make the V/G value constant, it is possible to pull a single crystal upwardly in an N region on the entire plane in which a defect region is removed from a radial direction of the single crystal.